This invention relates generally to electric propulsion systems for traction vehicles and, more particularly, to a method and apparatus for detecting and correcting for vehicle stall conditions.
An electric propulsion system for a traction vehicle, such as a large haulage truck, typically comprises a prime mover-driven electric generating means for supplying electric power to a pair of high-horsepower electric traction motors respectively connected in driving relationship to a pair of wheels on opposite sides of the vehicle. The prime mover is commonly a diesel engine, and the traction motors are generally adjustable speed, reversible direct current (d-c) electric motors. A vehicle operator controls the vehicle speed and direction of travel, i.e., forward or reverse, by manipulation of a speed control pedal and a forward-reverse selector switch. This speed control pedal is adapted to control the engine speed (rpm) which determines the power output of the generating means, thus varying the magnitude of the voltage applied to the traction motors.
Deceleration of a moving vehicle is accomplished by releasing the speed control pedal and either allowing the vehicle to coast or activating its mechanical or electrical braking system. In the electric braking mode of operation, the motors behave as generators, and the magnitude of the voltage generated across the armature windings of each motor is proportional to the rotational speed and the field excitation current of the motor. Dynamic braking resistor grids are connected across the armatures of the respective motors to dissipate the electric power output of the motors during electric braking. The average magnitude of current in each resistor grid is a measure of the braking effort of the associated motor.
During operation of such a traction vehicle, it is not unusual for the vehicle to become stalled. The term "stall" is used in its ordinary sense, i.e., to come to a standstill or stop. Two types of stalling are commonly encountered. A first type occurs when one driven wheel loses traction and begins to spin or slip while the other driven wheel stops rotating. This type of vehicle stalling may be referred to as a spin and/or stall condition. A second type of stalling occurs when both driven wheels are stalled, i.e., neither wheel is able to rotate. The first type of stalling may occur, for example, when one of the driven wheels encounters muddy terrain while the other remains on firm ground and the vehicle is so heavily loaded that the wheel on firm ground is unable to propel it. The second type of stalling may occur when a loaded vehicle is trying to ascend a slope or hill.
The electrical power system in most of these traction vehicles is arranged such that the d-c electric traction motors are connected in a series circuit to the vehicle-mounted power source, i.e., an electric generator. When one of the wheels loses traction and begins to spin, the counter electromotive force (CEMF) across the motor driving the spinning wheel increases, and consequently current in the motor pair will decrease. With less current, the non-spinning motor developes less torque, and the vehicle is more likely to stall. While increasing the generator output can somewhat compensate for the reduced power to the non-spinning wheel, available power is limited by both the generator capacity and the commutation or breakdown voltage and current limits of the motors. Thus, it is desirable to provide a method and apparatus for transferring power from the spinning to the non-spinning wheel without exceeding the generator and motor electrical limits.
When a traction vehicle encounters a total stall condition, i.e., neither of the driven wheels is able to rotate, the associated traction motors exhibit relatively low impedance resulting in rapidly increasing motor currents. Very large currents can overheat and damage the commutator bars and other parts of the motors. The period of time during which a non-rotating motor can remain energized without damage varies with the amount of electrical energy being supplied. However, it is desirable to keep the motors energized for as long as possible to maximize the chances of recovering from the stalled condition. It is therefore desirable to provide a method and apparatus for detecting a stall condition and for controlling current to the motors in a manner to attempt correction of the stall without risking damage of the motors.